PM: 'Deeply shocking' British citizen could be involved

US journalist James Foley's murder, apparently at the hands of a British Islamic State (IS) fighter, shows clearly the depravity and violence in Syria and Iraq - the killing filmed and released as propaganda by IS.

The footage is too shocking to show.

You may find some of the details distressing in this report by ITV News Deputy Political Editor Chris Ship:

David Cameron has ordered the security services to try to identify the killer.